For objects of such central importance to the franchise, the powers of the Infinity Stones in the Marvel Cinematic Universe remain somewhat ill-defined. Even a close examination of what the Infinity Stones have done in MCU movies thus far results in little more than a vague understanding of their abilities - aside from their enormous potential for universal upheaval.
The Infinity Stones have been a part of the MCU longer than some of best Avengers, with the Space Stone appearing in the post-credits scene of 2011’s Thor. They aren't referred to by name until Thor: The Dark World, but their overall importance to the MCU movies expanded in a hurry - or, one could say, in a snap.
The first appearance of an Infinity Stone in the Marvel Cinematic Universe occurs in the franchise’s fourth film, 2011’s Thor. The Stones themselves don’t play much of a role in Thor’s plot, though it is eventually hinted that some of Asgard’s glory can be attributed to Odin’s past dealings with the cosmic objects.
The Space Stone appears in the post-credits scene, where Dr. Erik Selvig is taken to a secure SHIELD facility to meet Nick Fury. The MCU’s chief spymaster introduces Selvig to the Tesseract - a mysterious cosmic cube that houses the Space Stone - and asks him to investigate it and its “untold power.” Selvig, seemingly influenced by Loki, readily agrees - setting the stage for Loki’s future schemes and placing this enigmatic item at the center of the franchise moving forward.
The Tesseract, which houses the Space Stone, makes its cinematic debut in Thor, but the full backstory of the object isn’t explored until the next film in the lineup, Captain America: The First Avenger. Unaware of its cosmic origins, the Red Skull and his HYDRA cronies uncover the Tesseract in 1942. It had previously been hidden by the Asgardians.
Back at HYDRA headquarters, the Red Skull and Dr. Arnim Zola do their best to harness the energy of the Tesseract for their tools of disruption - though they don't uncover a real understanding of the Infinity Stone hidden within the cube.
Their ultimate invention is the Valkyrie airship, which is powered by the Tesseract and with which they intend to strike the United States. Steve Rogers takes the craft down before it reaches North America, and during the confrontation, the Red Skull lays his hands upon the Tesseract and disappears.
While at first it might seem like the cube simply disintegrated the Skull, it’s revealed in Avengers: Infinity War that it teleported him across galaxies to the planet Vormir to serve as the keeper of the Soul Stone - the first hint of a “consciousness” among the Infinity Stones.
The Tesseract finally reveals its true nature in 2012’s The Avengers when it becomes active while under SHIELD observation and opens up a portal that brings Loki to Earth. Though it’s not revealed at the time, Loki has the Mind Stone hidden within his Thanos-gifted scepter, and he immediately uses it to control Clint Barton and other SHIELD agents.
After Captain America and Iron Man temporarily capture Loki, his Mind Stone-empowered scepter continues to work its magic, subtly influencing the Avengers to fight among themselves. Later, when Loki escapes, he uses the scepter to finish off Phil Coulson.
The similarly enthralled Erik Selvig uses his research with the Tesseract to tap into its space-related abilities, opening up an enormous portal in the sky above Manhattan for the Chitauri forces to pour through. Black Widow’s use of Loki’s discarded scepter closes the portal, demonstrating the power one Infinity Stone can have on another. The Tesseract is returned to Asgard for safekeeping, while Loki’s scepter - which no one seems to realize contains the Mind Stone - remains in SHIELD’s possession until HYDRA nabs it.
The events of Thor: The Dark World center around another cosmic MacGuffin - the Aether - which turns out to be the Reality Stone. The film reveals just how long these objects have been around with Malekith having used the Aether's power thousands of years ago in an attempt to wipe out the Nine Realms.
Back in the present day, Jane Foster rediscovers the Aether and finds herself infected with its energy. After Malekith takes on the Aether himself, he tries to use its reality-bending powers to plunge the universe into eternal darkness. He very nearly succeeds, demonstrating the might of the Infinity Stones.
After Malekith is defeated, Asgardians Sif and Volstagg bring the Aether to the Collector for safekeeping - and for the first time in the MCU canon, the word “Infinity Stone” is used. This post-credits scene also features the Collector muttering to himself about the necessity of locating the other five stones - foreshadowing Thanos's eventual quest.